Your K-50's AF detector has a focus sensitivity of f/5.6 for all points. What that means is that the system will focus as well at f/5.6 maximum aperture as at f/1.8 despite the greater DOF at f/5.6. This is typical for consumer-level dSLRs regardless of brand. This works fine for lenses having modest maximum aperture, but may yield a fair number of OOF results when shooting wide open at f/2.8 and faster. The OOF results will be about evenly split between front/back focus.

Another thing to consider is that mechanical precision in the focus mechanism is poorer at distance than close up (one step of the motor has greater effect). Focus testing is generally done at about 20x or 30x the lens focal length unless some sort of internal focus is present requiring tests at both near and far. Your DA 50/1.8 is not such a lens. Testing should also be done using both near-to-point and far-to-point to account for mechanical lash.

You may want to attempt manual focus with magnified live view as a check. If you have similar issues with softness, the problem is not with the AF system.

Tuned my da 50 for my k5 yesterday. Used the mtfmapper software (free software) to generate a focus graph. All my lenses consistently shifted focus back when closing the apperture ... Took lots of shots to confirm. Only worked with PEF files DNG failed for some reason.

From previous experience distance also matters. You might require different adjustments for different distances to subject.

Alright - sorry to keep going with this but it seems there's still a very slight soft front focus at 1.8 when at -10. I could live with it, but I know that in the back of my head I'll always worry about potential "focus drift" or something. Let's see how support is!

I've never read anything about the focus drift on any of Pentax's cameras except the K-5. I own a K30 which is the same guts as the K50 in a different body - and mine has been very consistent and reliable. If you had the opportunities to buy one of those extended warranties from Pentax for $20-30 then that should cover them servicing it to work with those lenses you have.
By the way you haven't mentioned which lens you are shooting with at 1.8? If it is the 50 1.8 - that is a great lens especially for the price, but I would bet that it is soft in that area wide open. I've shot with that lens on boy my K5s and K5II and K30 and K-X and all had the same result wide open. 2.0 for that lens and the FA 50 1.4 is perfect for good blur and sharpness.
For what it is worth.

I've never read anything about the focus drift on any of Pentax's cameras except the K-5. I own a K30 which is the same guts as the K50 in a different body - and mine has been very consistent and reliable. If you had the opportunities to buy one of those extended warranties from Pentax for $20-30 then that should cover them servicing it to work with those lenses you have.
By the way you haven't mentioned which lens you are shooting with at 1.8? If it is the 50 1.8 - that is a great lens especially for the price, but I would bet that it is soft in that area wide open. I've shot with that lens on boy my K5s and K5II and K30 and K-X and all had the same result wide open. 2.0 for that lens and the FA 50 1.4 is perfect for good blur and sharpness.
For what it is worth.

I think it could be the softness. I used the 50 1.8 for a while and I thought I had a focusing problem, but it is soft wide open. Stop it down into the 2.4-2.8 range and see how much it improves. Also try it on Live View to compare the images between that and viewfinder focus. Your Live View should be dead on, and at 1.8 you should be able to see how it should look. So if it is soft there, the lens is a bit soft wide open.

I really love the 50 1.8, its rendering and color and sharpness (stopped down) are excellent for a sub $100 prime. However, it is the least expensive Pentax lens sold. Hope this helps a bit.